Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 31 May 2022 10:59:10 -0700
Message-Id: <20220531175910.890307-1-samitolvanen@google.com>
Mime-Version: 1.0
Subject: [PATCH] cfi: Fix __cfi_slowpath_diag RCU usage with cpuidle
From:   Sami Tolvanen <samitolvanen@google.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
        Sami Tolvanen <samitolvanen@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

RCU_NONIDLE usage during __cfi_slowpath_diag can result in an invalid
RCU state in the cpuidle code path:

  WARNING: CPU: 1 PID: 0 at kernel/rcu/tree.c:613 rcu_eqs_enter+0xe4/0x138
  ...
  Call trace:
    rcu_eqs_enter+0xe4/0x138
    rcu_idle_enter+0xa8/0x100
    cpuidle_enter_state+0x154/0x3a8
    cpuidle_enter+0x3c/0x58
    do_idle.llvm.6590768638138871020+0x1f4/0x2ec
    cpu_startup_entry+0x28/0x2c
    secondary_start_kernel+0x1b8/0x220
    __secondary_switched+0x94/0x98

Instead, call rcu_irq_enter/exit to wake up RCU only when needed and
disable interrupts for the entire CFI shadow/module check when we do.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 kernel/cfi.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/kernel/cfi.c b/kernel/cfi.c
index 9594cfd1cf2c..08102d19ec15 100644
--- a/kernel/cfi.c
+++ b/kernel/cfi.c
@@ -281,6 +281,8 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
 static inline cfi_check_fn find_check_fn(unsigned long ptr)
 {
 	cfi_check_fn fn = NULL;
+	unsigned long flags;
+	bool rcu_idle;
 
 	if (is_kernel_text(ptr))
 		return __cfi_check;
@@ -290,13 +292,21 @@ static inline cfi_check_fn find_check_fn(unsigned long ptr)
 	 * the shadow and __module_address use RCU, so we need to wake it
 	 * up if necessary.
 	 */
-	RCU_NONIDLE({
-		if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
-			fn = find_shadow_check_fn(ptr);
+	rcu_idle = !rcu_is_watching();
+	if (rcu_idle) {
+		local_irq_save(flags);
+		rcu_irq_enter();
+	}
+
+	if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
+		fn = find_shadow_check_fn(ptr);
+	if (!fn)
+		fn = find_module_check_fn(ptr);
 
-		if (!fn)
-			fn = find_module_check_fn(ptr);
-	});
+	if (rcu_idle) {
+		rcu_irq_exit();
+		local_irq_restore(flags);
+	}
 
 	return fn;
 }
-- 
2.36.1.255.ge46751e96f-goog